Patent Application
20140064030
1. Field of the Invention
The present invention relates to an object information acquiring apparatus.
2. Description of the Related Art
One method of obtaining an optical characteristic value in a living organism is photoacoustic tomography (PAT) which utilizes a property of ultrasonic waves of being less scattered than light in a living organism (Non Patent Literature 1: M. Xu, L. Wang “Photoacoustic imaging in biomedicine”, Review of scientific instruments, 77, 041101(2006)). When pulsed light generated by a light source is irradiated on a living organism, the light propagates in the living organism while being diffused. When an absorber included in the living organism absorbs propagated light, an acoustic wave such as an ultrasonic wave is generated due to a photoacoustic effect. By receiving the ultrasonic wave with a probe and analyzing the received signal, an optical characteristic value distribution and, more particularly, a light absorption density distribution in the living organism can be obtained.
According to M. Xu, L. Wang “Photoacoustic imaging in biomedicine, Review of scientific instruments, 77, 041101(2006), sound pressure P of an ultrasonic wave obtained from an absorber in a living organism by light absorption according to PAT can be expressed by Equation (1) below.
P=Γ·μ
a·φ (1)
In Equation (1), Γ denotes the Gruneisen coefficient which is an elasticity characteristic value obtained by dividing a product of a coefficient of volumetric expansion β and the square of the speed of sound c by specific heat Cp. μa denotes an absorption coefficient of the absorber and φ denotes a light flux that is absorbed by the absorber.
As is apparent from Equation (1), sound pressure of an ultrasonic wave according to PAT is proportional to an amount of light that reaches the object. Therefore, in order to obtain a strong signal, the amount of the light that is irradiated on the object must be increased.
On the other hand, maximum permissible exposure (MPE) as a maximum value of irradiation density that is applied to a living organism is specified as a safety-related standard regarding lasers (JISC 6802). Uniform illumination is required in order to increase the amount of light irradiated on a living organism while taking MPE into consideration.
In addition, with PAT, in order to conduct a measurement over a wide range of a living organism, the living organism is desirably scanned by an illuminating unit and a receiver. In the case of a large light source such as a solid-state laser, it is difficult to perform a scan using the light source itself. Therefore, preferably, light emitted from the light source is transmitted by an optical fiber and a scan is performed using an outputting unit of the optical fiber. When the amount of light is large and light cannot be transmitted by a single optical fiber, a bundle fiber that is a bundle of optical fibers is preferably used.
Since a property of a bundle fiber is that light spreads after being outputted, a peripheral part has a smaller amount of light than a central part. While uniform beams can conceivably be produced by using a lens or the like between the bundle fiber and the living organism, this disadvantageously increases apparatus size. In particular, with PAT which uses a hand-held illuminating unit, an operating unit is desirably constituted by a minimum number of parts in order to reduce weight.
In the field of image display apparatuses, an example is disclosed in which an outputting unit of a bundle fiber branches into a plurality of equal sub-bundles (Patent Literature 1: Japanese Patent Application Laid-open No. 2002-214707). Accordingly, light emitted from a light source is transmitted by an optical fiber whose outputting unit branches into a plurality of sub-bundles, and by arranging the sub-bundles at positions corresponding to respective display elements and illuminating the display elements, a variation in amounts of light among the display elements is reduced and a high-quality image is obtained.
The method disclosed in Japanese Patent Application Laid-open No. 2002-214707 has a problem in that when the outputting unit of the fiber and an illuminated region are close to each other, a dark portion that is not illuminated is created between regions illuminated by the sub-bundles and uniform illumination cannot be achieved. In addition, when the outputting unit of the fiber and an illuminated region are far from each other, there is a problem that overlapping of light is stronger in a central part than in a peripheral part of an illuminated region and uniform illumination cannot be achieved.
The present invention has been made in consideration of these problems and an object thereof is to improve uniformity of light irradiated from a bundle fiber on an object.
The present invention provides an object information acquiring apparatus comprising:
The present invention also provides an object information acquiring apparatus comprising:
The present invention also provides an object information acquiring apparatus comprising:
The present invention also provides an object information acquiring apparatus comprising:
According to the present invention, the uniformity of light irradiated from a bundle fiber onto an object can be improved.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. However, dimensions, materials, shapes, relative arrangements, and the like of components described below are to be modified as appropriate depending on configurations and various conditions of apparatuses to which the invention is applied, and are not intended to limit the scope of the invention to the following description.
The present invention can be favorably applied to an illuminating apparatus which irradiates light using a bundle fiber. In particular, by applying the present invention to an object information acquiring apparatus including such an illuminating apparatus, the uniformity of light irradiated on an object can be improved. Such object information acquiring apparatuses include apparatuses which irradiate light (electromagnetic waves) on an object and receive acoustic waves generated inside the object, and which acquire object information in the form of image data.
Object information that is acquired by an apparatus utilizing a photoacoustic effect represents a generation source distribution of acoustic waves generated by light irradiation, an initial sound pressure distribution inside an object, or a light energy absorption density distribution, an absorption coefficient distribution, or a concentration distribution of a tissue-forming substance that is derived from the initial sound pressure distribution. Examples of a concentration distribution of a substance include an oxygen saturation distribution and an oxygenated/reduced hemoglobin concentration distribution.
Acoustic waves as described in the present invention are typically ultrasonic waves and include elastic waves that are referred to as sound waves, ultrasonic waves, or acoustic waves. An acoustic wave generated by a photoacoustic effect is referred to as a photoacoustic wave or a light ultrasonic wave. An acoustic detector (for example, a probe) receives acoustic waves generated or reflected inside an object.
Hereinafter, an object information acquiring apparatus according to an embodiment of the present invention and, in particular, a configuration of an illuminating apparatus that is a feature of the present invention will be described.
(Light Source)
When the object is a living organism, the light source irradiates light with a wavelength that is absorbed by a specific component among components that constitute the living organism. The light source may be integrally provided with the living organism image acquiring apparatus according to the present embodiment, or the light source may be separated and provided as a separate body. A pulse width is preferably around 10 to 50 nanoseconds in order to generate photoacoustic waves in an efficient manner.
While a laser capable of producing a large output is favorably used as the light source, a light-emitting diode, a flash lamp, or the like can be used instead of a laser. Various lasers can be used including a solid-state laser, a gas laser, a dye laser, and a semiconductor laser. Timing, waveform, intensity, and the like of irradiation are controlled by a light source controller.
Moreover, the light source controller may be integrated with the light source. The wavelength of the light source used in the present invention is desirably a wavelength that allows light to propagate to the inside of an object. Specifically, when the object is a living organism, the wavelength is equal to or more than 500 nm and equal to or less than 1200 nm.
(Bundle Fiber)
The bundle fiber is constructed by bundling together a plurality of optical fibers. An optical fiber has a core made of silica glass or the like. Specifically, for example, the core has a diameter of 190 μm. Light from the light source enters from an incidence section of the bundle fiber and is transmitted by the respective fibers.
An output side of the bundle fiber branches into a plurality of sub-bundles. Each sub-bundle is provided with an outputting edge for outputting the transmitted light. The outputting edge of each sub-bundle is fixed so that end faces of the outputting edges are aligned in an outputting unit. A plane of the outputting unit on which sub-bundle end faces are aligned form an outputting plane. Light outputted from the plurality of sub-bundle end faces on the outputting plane overlap each other to illuminate an object. A sub-bundle in a central part corresponds to the first sub-bundle and a sub-bundle in a peripheral part corresponds to the second sub-bundle.
When the same sub-bundle is aligned at equal intervals, since overlapping of light in the peripheral part is weaker than overlapping of light in the central part, uniformity is low. In consideration thereof, in order to increase the amount of light in the peripheral part to improve uniformity, the bundle fiber according to the present invention is configured such that the density of an optical fiber in the peripheral part (the sub-bundle 103a) is greater than the density of an optical fiber in the central part (the sub-bundle 103b) in the outputting unit. However, if the outputting edge of a sub-bundle and the light-irradiating plane are too close to each other, light does not overlap and the effect of the present invention cannot be produced.
In consideration thereof, a range in which the effect of the present invention can be produced will be described with reference to
In addition,
L·tan θ≧0.76p−0.63W (2)
In consideration thereof, the following examples have been configured so as to satisfy this condition.
(Object Information Acquiring Apparatus)
When a living organism is the object, the object information acquiring apparatus according to the present embodiment is a living organism image acquiring apparatus which calculates information from the living organism as image data. However, the object information acquiring apparatus may be used on an object other than a living organism. As basic hardware components, the object information acquiring apparatus comprises a light source, a bundle fiber, a probe which receives acoustic waves, and a processor which performs image reconstruction.
Pulsed light emitted from the light source is transmitted by the bundle fiber and irradiated on a living organism. When a part of the energy of light propagated inside the living organism is absorbed by a light absorber (which eventually becomes a sound source) such as blood, an acoustic wave is generated by a thermal expansion of the light absorber. The acoustic wave is received by the probe and becomes an electric signal which is then transmitted to the processor. Based on the electric signal, the processor generates optical characteristic value distribution information from the living organism (image reconstruction). The optical characteristic value distribution information is not limited to a particular format. A format of the optical characteristic value distribution information can be arbitrarily determined based on a measurement objective, an apparatus configuration, and the like including two-dimensional and three-dimensional formats.
(Probe)
The probe receives an acoustic wave generated on a surface of the living organism or inside the living organism due to the pulsed light. Therefore, the probe is capable of converting the acoustic wave into an electric signal (received signal) that is an analog signal. Any kind of probe may be used including a probe using a piezoelectric phenomenon, a probe using resonance of light, and a probe using a variation in capacitance as long as the probe is capable of receiving acoustic wave signals. Favorably, the probe according to the present embodiment typically has a plurality of receiving elements arranged one-dimensionally or two-dimensionally. Using such multidimensionally-arranged elements enables acoustic waves to be simultaneously received at a plurality of locations to reduce measurement time. When there is only one receiving element, a scan may be performed using the probe to receive acoustic waves at a plurality of positions.
The object information acquiring apparatus desirably comprises a converter which converts an electric signal obtained by the probe from an analog signal to a digital signal and a circuit which amplifies the electric signal. When a plurality of received signals is obtained from the probe, desirably, a plurality of signals is simultaneously processed. Accordingly, the time required to form an image can be reduced. The converted signal is stored in a memory.
(Processor)
The processor uses the signal stored in the memory to form data related to optical characteristic value distribution information such as an initial sound pressure distribution of acoustic waves. For example, time-domain back projection can be used to form the optical characteristic value distribution. For example, an information processing device or a circuit which is operated by a program can be used as the processor.
In the present example, an example where the density of fibers contained in a sub-bundle in a central part is smaller than the density of fibers contained in a sub-bundle in a peripheral part will be described.
A diameter of a core of a fiber strand is 190 μm. In the incidence section, fiber strands are approximately hexagonal close-packed. The outputting unit of the bundle fiber 302 branches into nine 1 mm [sq] sub-bundles 303. The nine sub-bundles are arranged in a 3×3 array at an equal pitch p of 6 mm between two adjacent sub-bundles.
A feature of the present example is that the numbers of optical fibers contained in the respective sub-bundles differ between the central part and the peripheral part. Specifically, if the number of fibers contained per unit area of the sub-bundle 303a in the peripheral part is 1, then the number of fibers contained per unit area of the sub-bundle 303b in the central part is 0.3. Light outputted from each fiber can be approximated to a Gaussian distribution. A measurement of a spread angle θ of light from a single optical fiber in the present example resulted in tan θ=0.1. In this case, an angle over which intensity drops from a maximum intensity to 1/e2 is defined as θ.
On the other hand, an example of a conventional illuminating apparatus as a target for comparison is shown in
A comparison of
While an example where square sub-bundles are arranged at equal intervals in a 3×3 array has been given in the description above, the present invention is not limited thereto. As an example, a 4×4 arrangement of square sub-bundles is shown in
In addition to sub-bundle arrangements with the same number of vertical and horizontal sub-bundles as shown in
In the present example, an example will be described which produces a more uniform illumination distribution by varying the density of optical fibers contained in sub-bundles depending on location even in the peripheral part. Specifically, when the sub-bundle array is square, the density of optical fibers contained in a sub-bundle corresponding to a side of the square is set lower than the density of optical fibers contained in a sub-bundle at a corner (vertex) of the square.
The outputting unit of the bundle fiber 502 branches into nine 1 mm [sq] sub-bundles in a similar manner to the first example. The nine sub-bundles are arranged in a 3×3 array at an equal pitch p of 6 mm between two adjacent sub-bundles.
A feature of the present example is that the numbers of optical fibers contained in the respective sub-bundles vary depending on positions of the sub-bundles. In other words, if the number of optical fibers per unit area of a sub-bundle 503a in the peripheral part and at a vertex of a square is five, then the numbers of optical fibers per unit area of a sub-bundle 503b on a side portion of the peripheral part and a central part 503c are, respectively, three and one. Light outputted from each optical fiber can be approximated to a Gaussian distribution. In the present example, a spread angle θ of light from a single optical fiber was tan θ=0.1. In this case, an angle over which intensity drops from a maximum intensity to 1/e2 is defined as θ.
As a target for comparison,
From the above, by varying the density of optical fibers contained in sub-bundles depending on locations of the sub-bundles even in the peripheral part as in the present example, a more uniform illumination distribution can be obtained. Moreover, while an example where an array of square sub-bundles is arranged in a square has been described in the present example, this example is not restrictive. In addition, the sub-bundles may be arranged in a hexagonal close-packed structure.
Examples of non-square arrangements of sub-bundles include polygonal sub-bundle arrangements such as a triangle or a hexagon. In such cases, the numbers of optical fibers per unit area of sub-bundles in a central part, a side part, and a vertex part of the polygon need only satisfy (central part)<(side part)<(vertex part).
In the present example, an example will be described in which the numbers of optical fibers per unit area of respective sub-bundles are the same and a uniform illumination distribution is obtained by varying areas of the respective sub-bundles according to position.
The outputting unit of the bundle fiber 602 branches into nine sub-bundles. While the number of optical fibers per unit area of the respective sub-bundles is the same, sizes of the sub-bundles vary depending on an arrangement of the sub-bundles and the closer to center, the smaller the sub-bundle. A size of a sub-bundle 603a near a vertex in the peripheral part is 1.2 mm [sq], a size of a sub-bundle 603b of a side part in the peripheral part is 1 mm [sq], and a size of a sub-bundle 603c in the central part is 0.7 mm [sq].
The illuminating apparatus according to the present example is advantageous in that the illuminating apparatus is easier to manufacture than a system where the number of optical fibers per unit area is varied as in the first and second examples.
Examples where an irradiation density or an area of an outputting edge varies among sub-bundles have been described for the first to third examples. In the present example, an example will be described in which sub-bundles with the same irradiation density or area are arranged so that density in a central part is lower than density in a peripheral part.
The outputting unit of the bundle fiber 702 branches into 21 sub-bundles 703 (1 mm [sq]). The 21 sub-bundles 703 are arranged as shown in
As a target for comparison,
As is apparent from a comparison of
Although an example where sub-bundles are arranged in a square shape has also been described in the present example, this arrangement is not restrictive.
Specifically, sub-bundles may be arranged in a polygonal shape such as a triangle or a hexagon. In such cases, the numbers of arranged outputting edges of sub-bundles in a central part, a side part, and a vertex part of the polygon need only satisfy (central part)<(side part)<(vertex part).
While an illuminating apparatus has been described in the first to fourth examples, in the present example, an example will be described where a uniform illumination distribution is realized without inserting a lens system between a bundle fiber and a living organism in an object information acquiring apparatus.
Light 904 outputted from a sub-bundle illuminates a living organism 906 that is held by a holding plate 905a on the side of the bundle fiber 902 and a holding plate 905b on the opposite side of the bundle fiber 902. Desirably, the holding plate 905a readily transmits light while the holding plate 905b readily transmits acoustic waves and has an acoustic impedance that is close to the acoustic impedance of a living organism. For example, the holding plate 905a may be made of acryl resin and the holding plate 905b may be made of polymethylpentene. In the present example, acryl and polymethylpentene both have a thickness of 10 mm. Acryl has a refractive index of 1.49. In order to secure an optical path length of 100 mm from the fiber outputting edge to the living organism in a similar manner to the first example, a distance from the fiber outputting edge to the acryl plate was set to 85.1 mm.
The illuminated light is diffused in the living organism 906, and an acoustic wave 908 is generated when the diffused light is absorbed by an absorber 907. The acoustic wave 908 is propagated in the living organism 906 that is an object, and a part of the acoustic wave 908 is received by a probe 909. A received signal 910 is sent to a processor 911 and optical characteristic value distribution information in the living organism is formed. The sub-bundles 903a and 903b and the probe 909 are movable in a two-dimensional direction that is parallel to the holding plate 905.
By adopting the configuration described above, a variation in light intensity distribution among different light-irradiating positions can be reduced in a similar manner to the first example. As a result, a photoacoustic signal can be obtained more efficiently.
While a system in which a holding plate is scanned by a sub-bundle and a probe has been described in the fifth example, in the present example, a hand-held object information acquiring apparatus in which a sub-bundle and a probe are manually moved on a living organism will be described.
When performing a measurement, the outputting unit 1004 is brought into contact with a living organism 1006 that is an object and light 1005 illuminates the living organism 1006. The illuminated light is diffused in the living organism 1006, and an acoustic wave 1008 is generated when the diffused light is absorbed by an absorber 1007. The acoustic wave 1008 is propagated in the living organism 1006, and a part of the acoustic wave 1008 is received by a probe 1009. In this case, the probe 1009 is integrated with the outputting unit 1004 and can be used to manually scan the living organism 1006.
While light irradiated on a living organism is desirably uniform as described earlier, in the case of a hand-held apparatus, the number of parts of an illuminating system is desirably minimized in order to reduce weight of an operating unit. Since the illuminating system described in the present example does not use an optical system for uniform illumination between the bundle fiber and the living organism, the illuminating system can be constructed using a minimum number of parts. Moreover, while a configuration which does not use a lens between an outputting unit and a living organism has been described in the present example, a more uniform optical system can be obtained by using an optical part such as a diffuser plate. In addition, a cover glass or the like can also be provided on an outputting edge of a bundle fiber.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2012-194164, filed on Sep. 4, 2012, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-194164 | Sep 2012 | JP | national |
